Anodes made of manganese oxides have been known for a long time and are disclosed, for instance, in U.S. Pat. Nos. 1,296,188 and 1,143,828. Such anodes have been used in the electrowinning of metals such as zinc, copper and nickel. For various reasons, such as the difficulties met with in forming them, such anodes are not suitable for commercial use, however. Another proposed electrode is described in U.S. Pat. No. 3,855,084, wherein titanium particles are cemented together with thermally-deposited manganese dioxide and a second or outer coating of electrodeposited manganese dioxide is provided thereon.
U.S. Pat. No. 3,616,302 describes an electrowinning anode, comprising a sandblasted titanium substrate coated with a thin intermediate layer of platinum, palladium or rhodium or their alloys, on which a relatively thick layer of manganese dioxide is electroplated.
U.S. Pat. No. 4,028,215 discloses an electrode which comprises a valve metal substrate, an intermediate semi-conductive layer of tin and antimony oxides and a top coating of manganese dioxide.
More recently, U.S. Pat. No. 4,072,586 proposed an electrode having a corrosion-resistant substrate coated with .beta.-manganese dioxide, chemideposited by thermal decomposition of an alcoholic solution of manganese nitrate, and activated by .beta.-ray irradiation or by the addition of up to 5% by weight of at least one metal from groups IB, IIB, IVA, VA, VB, VIB, VIIB and VIII of the Periodic Table, excluding the platinum group metals, gold and silver. The corrosion-resistant substrate was optionally provided with a thin porous intermediate coating, such as a valve metal or a platinum group metal or oxide thereof, and the activated manganese dioxide optionally contained up to 20% by weight of silicon dioxide, .beta.-lead dioxide or tin dioxide as stabilizer.